Well drilling assembly



Dec. 8, 1964 w. s. CRAKE WELL DRILLING ASSEMBLY Filed Sept. 11, 1961 uw um M M .H lll V n .n n

FIG.

INVENTORI FIG.

United States Patent 3,160,213 WELL DRILLING ASSEMBLY Wilfred S. Cralte, Houston, Tex., assigner to Shell Oil This invention relates to the drilling of wells in earth formations such as oil and gas wells, and pertains more particularly to a well drilling apparatus employing a unique drill collar designed to reduce the tendency of a drill bit to wander from a vertical line, especially when drilling through steeply dipping formations.

In drilling oil wells, one or more heavy drill collars are generally interposed between the drill bit and the rest of the drill string in order to concentrate a predetermined heavy weight on the bit so as to produce the best rate of drilling. Drill collars generally take the form of thickwalled pipe sections which may be or more feet in length and are provided with a central bore therethrough for'circulating a drilling fluid to the bit. The drill collars, together with the rest of the drill string secured to the top of the uppermost collar, forms an elongated steel column which pushes the drill bit downwardly against the formation. Because of the length and flexibility of this elongated steel column of drill pipe and collars, the steel column bends slightly so as to contact the wall of the well bore at several points between the bottom of the well and the surface ofthe ground. The spacing between adjacent points of contact between the flexible drill string and the borehole wall increases as the points of contact get higher in the borehole due to the fact that there is less weight of drill pipe being exerted downwardly at the higher points of contact.

Thus, the lowermost part of the drill string or drill collar between the bit and the first point at which the drill collar contacts the wall of the borehole, will form an angle to the true axis of the borehole. If the lowermost portion of the borehole should happen to be Vertical, which is seldom the case, the axis of the lowermost portion of the drill collar would lie at a small angle to the vertical since the wall of the drill collar would lay against the wall of the borehole at some point above the bit. In the more normal case, where the axis of the well bore was, say, 2 to the vertical, the drill collar would lay against the lower side of the borehole and its axis may form an angle of, say, 3 to the vertical. In either case there is a tendency for the drill bit to drill oii at an angle to the axis of the borehole thus causing a deviation which results in a crooked hole. When drilling operations are carried out with the drill collar touching the wall by bending a small amount, the axis of rotation of the drill collar remains in the center of the collar as it rotates around its bent axis, thus pointing the bit at a slight angle to the true bore of the well. As drilling operations are continued in the well, this angle causes the amount of horizontal deflection of the well from the vertical to increase. It is presently known that the deection of the drill string and bit can be decreased by reducing the weight on the drill column or by utilizing at least one full-holediameter stabilizer at a suitable height above the bit so that the portion of drill collar below the stabilizer acts as a pendulum which tends to exert a force causing the bit to come back toward the vertical again. However, with all the known aids and techniques of drilling presently available, crooked well bores are still being drilledl practically all of the time.

It is therefore a primary object of the present invention to provide a well drilling assembly adapted to counteract any `tendency of a drill bit to drill off at an angle fromthe desired vertical borehole.

It is known that, when drilling through formations which are inclined or dip at an angle to the horizontal, a rotating drill bit tends to drill up-dip, or tries -to drill in a direction normal to the formation dip planes. The angle at which formations dip is one of the major forces tending to produce a deilection of a drill bit which is normally inclined to the two axis of a borehole at a slight angle as described hereinabove. The direction of inclination of a borehole is predominantly lixed toward a plane normal to the formation dip, and thus the direction in which the axis of the drill collar is pointed is also controlled and iixed. Thus, continued drilling through a dipping formation results in a borehole with increasing deflection until a point is reached where it is necessary to employ various types of tools well known to the art to head the bit back in a vertical direction again.

It is therefore another objecttof the present invention to provide a well drilling apparatus adapted to constantly correct any deviation of the borehole from the vertical so as to eliminate the need for installing special equipment in the borehole to bring a crooked hole back to normal.

A further object of the present invention is to provide a well drilling assembly in which any tendency to deflect from the vertical when drilling through sloping or dipping formations is greatly reduced or substantially eliminated.

These and other objects of this invention will be understood from the following description taken with reference to the drawing, wherein:

FIGURE 1 is a longitudinal diagrammatic view illustrating the lower end of a drill string with drill collars positioned in a well;

FIGURES 2 and 3 are diagrammatic views illustrating two arrangements of the drilling assembly of the present invention when positioned in a well;

FIGURES 4, 5 and 6 are horizontal views of Various modifications of the drill collar of the present invention;

and,

FIGURES 7, 8 and 9 are cross-sectional views of FIG- URES 4, 5 and 6, respectively, taken along the line A-A.

Referring to FIGURE 1 of the drawing, a drill string is illustrated as being made up of a plurality of sections of drill pipe 11, 12, 13 and 14 and a pair of drill collars 15 and 16 with a bit 17 secured to the lower end of the lowermost drill collar 16. If desired the drill collars may be provided, at one or more levels, with a series of radially-extending rib elements or any other suitable type of centralizer or stabilizer device which is fastened to the outside of the drill collars so as to reduce the amount of play between the outer surface of the drill collars and the wall of the borehole 20. In the design of the centralizer or stabilizer device illustrated in FIGURE l, a series of radially-extending ribs 2l are welded tothe outer surface of the drill collar 15, preferably in a diagonal direction. When weight is applied to the lower end of the drill string during drilling operations, the drill string would lay against the wall of the borehole at several points. Since the stabilizer ribs 21 project outwardly from the collar, they would probably form one point of Contact for the drill string with the borehole wall, so that y the section of the drill string below the `stabilizer 21 would assume an angle to the vertical axis of the borehole. Continued drilling of the hole would cause the borehole to deviate from the Vertical at an angle assumed by the drill collars.

In FIGURE 2 the drill collar 22 of the drilling assembly of the present invention is designed to have its axis temporarily locked at an angle under load conditions by providing a section modulus of the collar 22 which is predominantly greater in one direction than in another. Any suitable means may be employed for reducing the section modulus of a portion of the drill collar so that the. collar i-s preferentially bendable in one direction. For example,

3 s illustrated in FIGURES 2 through 9, the section-moduls-reducng means takes the form of a portion of the ollar having a reduced wall thickness which is formed in manner so that the Wall is thicker on one side than the ther. For example, in FIGURE 2 the wall thickness of rill collar- 22 is reduced by cutting Va slot 23 into the all of the collar from the outside thereof, preferably on line normal to the axis of the collar. When column Jading is applied to the top of drill collar 22, the collar 2 will be caused to bend at the slot 23 since the section modulus of the collar is predominantly greater in one irection than in a direction taken at 180 to the slot. As ong as a. load is applied to the topo-f the drill collar 22, s axis will remain locked -at a slight angle from the ver- Iical, as illustrated in FIGURES 2 .and 3, although it is to e understood thatthe illustrated `angle in these figures is reatly exaggerated. Y

Since the side of the drill collar 22 diametrically op, rosite the slot 23 will bear against the borehole wall 24, uitable bearing means or wear reducing means 25 are vrovided. For example, one form lof bearing means or lrear reducing means may take the for-m of a hard-sur aced area of some suitable material such as tungsten caride which would reduce the wear of that portion of the uter wall of the collar 22 in contact with the wall of the orehole Alternatively, one or more rollers rotating on substantially vertical axis could be positioned on the uter wall of the collar 22 opposite the slot 23.

The slot 23 of collar 22 is shown in greater detail in iIGURES 4 and 7. The exact position of the slot 23 .long` the collar 22 rand the size and depth of the slot 23 Vould vary depending upon the load conditions, the dirneter of the collar, the thickness of the wall of the colar, the material of which the collar is made, etc. For a )it drilling an 8% inch ihole, a 7% inch drill collar having inch bore may be employed. Instead of using a single lot 23, Ia second slot 26 in the opposite wall may be cut to tllow bending of the collar 22 under .a lighter load. As an xample, the slot 23 of the71/2 inch drill collar may be inches deep while the slot 26 positioned diametrically )pposite may be one inch deep. For this particular size lrill collar the slots 23 and 26 are preferably positioned tbove the bit a distance less than 20 times the diameter of be hole being drilled. The distance between the bit 17 md the slots 23 and 26 determines the angle of the drill :olla-r 22 t-o the normal borehole wall when the axis of the lrill collar is locked so that the portion of the outer wall tdjacent the smaller slot 26 contacts the wall of the borelole. Thus, an angle of O degree 19 minutes is formed vetween the axis of the drill collar and the axis of the hole when the slotted portion of the collar contacts the wall 1t va height l0 feet above the bit. When the height is 8 eet, the angle is degree, 23 minutes. It is to be understood that instead of cutting the slot 23 n the outer wall of the drill collar, a at or other suitable :onliguration may Ibe milled in the wall of the collar so 1s to preferentially reduce the wall thickness on one side :hereof so that the collar will -bend .at the point of section modulus. In the event that the coll-ar 28 (FIGURE 6) has an extremely thick wall, a portion 29 of the collar may be reduced in diameter with a slot 3i) cut in one wall to further reduce the section modulus.

When the drill collar 22 of FIGURE 2 is columnloaded from above so that its axis is locked at a predetermined angle from the axis of the borehole, upon rotation of the drill string the ,axis of the collar 22 below the slot 23 describes a conical surface around the bit center of rotation and thus causes the bit to directionally drill att-he same angle to all point-s on the compass as it rotates. With the bit directionally-drilling to :all points of the'compass, deflection of the bit which would normally take place is substantially reduced and in some formations eliminated. To aid further in reducing any deflection, therwell drilling .assembly may be provided with one or more centralizers or stabilizers of -any suitable design which are d. diagrammaticallly illustrated in FIGURE 2 Vas element 32. The stabilizer for centralizer 32 is positioned above the bit, preferably at a distance equal to twice Vthe distance between the bit and the slot 23.

In order to provide la more positive and controlled bent axis to the drill collar 22 (FIGURE 2), the drill collar or collars lpositioned above the collar 22 may be of the fullypacked-hole type of drill collar which has a diameter substantially equal to that of the rbit, or having a diameter that is only Ma yor 1A of an inch smaller than the diameter of the borehole. One form of a drill collar of this type is illustrated as collar 15 in FIGURE l which is provided with spiral elements or lblades 21 which would reduce the vibration in the upper collars which is caused by the bent axis of the lowermost collar. Instead of a round drill collar being used, the upper collar or collars 1S may be square in cross section with the diagonal dimension being slightly less than'the borehole diameter. By such means, adequate annular clearance for cuttings removal is provided and still a packed hole is maintained.

Another form of the well drilling assembly of the present invention is illustrated in FIGURE 3 as compris ing a drill collar'33 having three pairs of slots 34 and 3S, 36 and 37 Iand 38 and 39 which are cut in the outer wall thereof in a manner illustrated by the slots 23 and 26 of FIGURE 5. A preferential bend direction is imparted to the 4collar 33 by making the deeper cuts 34, 36 and 38 on the side lof the collar toward which it is desired to have the collar bend. By making two consecutive cuts 34 and 36 deeper on one side a sort of crank angle is formed with a flat portion of the collar between cuts and 37 contacting the borehole wall to -act as an elongated bearing surface thereagainst which is 'provided with suitable hard surfacing. The greater the weight on the column of drill string and the collars attached thereto, the greater will the thrust on `the section in contact with the borehole between slots 35 and 37, but due to the length chosen between the cuts or slot, .all ofthe sections of drill collar on each side of the slots are so stiff that insignificant bending takes place in them. In this case the distance between the bit and slot 34 is 8 feet, between slots 34 and 36 there is a 6 foot interval with 8 foot intervals between slots 36 and 39 and between slot 39 and the stabilizer or centralizer 32.. In this case, with an 11 inch outside diameter drill collar 33 positioned in a 12% inch borehole 4t) the section of the collar between the bit 17 and the rst slot 34 assurnes an angle of about 0 degree 24 minutes with the axis of the borehole 40. The horizontal thrusts at the slots and the bit, with 100,000 lbs. column load on the drill :st-ring, are 700 lbs. lat each point, and the bearing load on the surface 41 parallel to the borehole wall is 1,400 lbs. from the thrust.

I claim as my invention:

1. Apparatus to minimize well deviation during drilling a well comprising a drill string `adapted to be positioned in `a Well, at least one thick-wall drill collar secured to the lower end of said drill string, and a drill rbit secured to the lower end of said drill collar and rotatable with said collar and said drill string, a portion of said drill collar being formed with reduced wall thickness only on one side thereof to reduce the section modulus of the collar directionally, whereby under load conditions the `axis of the drill collar lbelow the portion of reduced section modulus becomes locked in one direction at an angle to the axis of the well whereby the angle formed in the collar sweeps around the wall of the well.

2. The apparatus of claim 1 wherein the portion of the reduced wall thickness comprises a horizontal slot cut partially into the wall of said collar from the outside and less than the wall thickness thereof. f

3. The apparatus of claim 1 including bearing means carried on the outer Wall of said drill collar adjacent the reduced Wall thickness portion thereof.

4. The apparatus of claim 3 wherein the bearing means comprises hard-surfacing material affixed to the outer surface of said drill collar.

5. The apparatus of claim 1 including stabilizer means secured to said drill string adjacent the top of said collar.

6. The apparatus of claim 1 wherein lsaid rdrill collar has a horizontal dimension substantially equal to the diameter of the well being drilled by the bit.

7. Apparatus to minimize well deviation during drilling a well comprising a drill string adapted to be positioned in a well, at least one thick-wall drill collar secured to the lower end of said drill string, and a drill bit secured to the lower end of said drill collar and rotatable with isaid collar and said drill string, a portion of said drill collar being formed with reduced wall thickness to reduce the section modulus of the collar directionally, the portion of reduced wall thickness being a pair of horizontal slots cut partially into the wall of said collar from the outside thereof, said slots being at points substantially diametrically opposite each other, one of Isaid slots being deeper than the other, whereby under load conditions the axis of the drill collar below the portion of reduced section modulus becomes locked in one direction at an angle to the `axis of the well whereby 6 the angle formed in the collar sweeps around the wall of the well.

8. Apparatus to minimize well deviation during drilling a well comprising a drill string adapted to be positoned in a weil, at least one thickJwall drill collar secured to the lower end of said drill string, and a drill bit secured to the lower end of said drill collar and rotatable with said collar and drill string, there being three axially-spaced portions of said drill collar being formed with reduced wall thicknesses to reduce the section modulus of `said collar directionally, whereby under substantially vertical loading conditions the axis of the collar is bent and locked at an angle to the drill string, two portions of the collar with reduced wall thickness nearest the bit being on the same side of the collar with the third portion being on the opposite side of the collar.

References Cited in the le of this patent UNITED STATES PATENTS 2,336,334 Zublin Dec. 7, 1943 2,336,338 Zublin Dec. 7, 1943 2,829,864 Knapp Apr. 8, 1958 2,841,366 Dunn July l, 1958 

1. APPARATUS TO MINIMIZE WELL DEVIATION DURING DRILLING A WELL COMPRISING A DRILL STRING ADAPTED TO BE POSITIONED IN A WELL, AT LEAST ONE THICK-WALL DRILL COLLAR SECURED TO THE LOWER END OF SAID DRILL STRING, AND A DRILL BIT SECURED TO THE LOWER END OF SAID DRILL COLLAR AND ROTATABLE WITH SAID COLLAR AND SAID DRILL STRING, A PORTION OF SAID DRILL COLLAR BEING FORMED WITH REDUCED WALL THICKNESS ONLY ON ONE SIDE THEREOF TO REDUCE THE SECTION MODULUS OF THE COLLAR DIRECTIONALLY, WHEREBY UNDER LOAD CONDITIONS THE AXIS OF THE DRILL COLLAR BELOW THE PORTION OF REDUCED SECTION MODULUS BECOMES LOCKED IN ONE DIRECTION AT AN ANGLE TO THE AXIS OF THE WELL WHEREBY THE ANGLE FORMED IN THE COLLAR SWEEPS AROUND THE WALL OF THE WELL. 